Merge legacy `fetchGit`-builtin with the generic `fetchTree`-function

The original idea was to implement a git-fetcher in Nix's core that
supports content hashes[1]. In #3549[2] it has been suggested to
actually use `fetchTree` for this since it's a fairly generic wrapper
over the new fetcher-API[3] and already supports content-hashes.

This patch implements a new git-fetcher based on `fetchTree` by
incorporating the following changes:

* Removed the original `fetchGit`-implementation and replaced it with an
  alias on the `fetchTree` implementation.

* Ensured that the `git`-fetcher from `libfetchers` always computes a
  content-hash and returns an "empty" revision on dirty trees (the
  latter one is needed to retain backwards-compatibility).

* The hash-mismatch error in the fetcher-API exits with code 102 as it
  usually happens whenever a hash-mismatch is detected by Nix.

* Removed the `flakes`-feature-flag: I didn't see a reason why this API
  is so tightly coupled to the flakes-API and at least `fetchGit` should
  remain usable without any feature-flags.

* It's only possible to specify a `narHash` for a `git`-tree if either a
  `ref` or a `rev` is given[4].

* It's now possible to specify an URL without a protocol. If it's missing,
  `file://` is automatically added as it was the case in the original
  `fetchGit`-implementation.

[1] https://github.com/NixOS/nix/pull/3216
[2] https://github.com/NixOS/nix/pull/3549#issuecomment-625194383
[3] https://github.com/NixOS/nix/pull/3459
[4] https://github.com/NixOS/nix/pull/3216#issuecomment-553956703

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index c3ee9bf43..28db8aa9c 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -134,7 +134,7 @@ std::pair<Tree, Input> Input::fetch(ref<Store> store) const
 
     if (auto prevNarHash = getNarHash()) {
         if (narHash != *prevNarHash)
-            throw Error("NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
+            throw Error((unsigned int) 102, "NAR hash mismatch in input '%s' (%s), expected '%s', got '%s'",
                 to_string(), tree.actualPath, prevNarHash->to_string(SRI, true), narHash->to_string(SRI, true));
     }